Transport and evolution of a pollution plume from northern China: A satellite‐based case study

Abstract

On 5 April 2005, during the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST‐AIRE) aircraft campaign, heavy loadings of SO2 (20 ppb near ground, 1–3 ppb at ∼2 km altitude) and dust with aerosol optical depth of ∼1 were measured over Shenyang, an industrialized city ∼600 km NE of Beijing. In this study, Ozone Monitoring Instrument (OMI) and MODIS satellite sensors are employed to look into this air pollution episode at a regional scale and to track the transport and evolution of the plume from China to the NW Pacific on the following days. A method is proposed to combine in situ measurements and trajectory tracer modeling with satellite observations to quantify the change in the SO2 mass during plume transport. We demonstrate that an air mass factor correction is needed for quantitative use of the OMI SO2 data, to account for the effects of the viewing geometry, the SO2 profile shape, and the aerosol/cloud interference on retrievals. The total SO2 loading of the plume decreased from ∼1.1 × 1011 g on 5 April to ∼5.0 × 1010 g on 7 April. The overall, e‐folding lifetime of SO2 in this plume, empirically derived from the rate of SO2 decay, was ∼2 days (range of 1–4 days). SO2 to sulfate conversion increased the aerosol optical depth by ∼0.1–0.4 near the center of the plume on 6 and 7 April, while the loss of primary dust particles reduced the aerosol loading of the plume by a similar amount. Simulations with a chemical transport model suggest similar loss of dust and formation of sulfate within the plume during transport. The method established in this study can be further developed and applied to study other episodes of pollution transport and their impact on weather and climate.